Introduction
The Casio FX-991EX can malfunction if the battery is discharged, even with solar power. Thankfully, replacing it is a simple job.
Tools
Parts
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Slot the new battery into the compartment. Ensure that the positive side (with writing) faces up...
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And the negative side faces the calculator's screen.
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Conclusion
If you need to reset the calculator after this, it's SHIFT+9 > 3 > =.
Otherwise, you're ready to crunch numbers again!
One comment
Just discovered during a lecture that my battery has become a spicy pillow - if you're leaving this in storage be careful and monitor it, lest it explode and/or warp the case irreparably.